Key words: Reptilia, Sphenodon, tuatara, feeding, nocturnal, behaviour, vision, infra-red-radiation.

Observations and Discussion

We have been keeping three young tuatara (Sphenodon punctatus) since May 1987 in the laboratory under natural light conditions for an investigation of tail regeneration and spinal cord ultrastructure (Alibardi & Meyer-Rochow. in prep.). The animals have been developing very well since they hatched in April 1987 and have been putting on weight at the rate of approx. 0.9 g/month on a mixed diet of slaters (mostly Oniscus sp.). amphipods (Talitrus spp.), mealworms (Tenebrio molitor), beetles (Chaerodes sp.), and flies (Musca sp.).

Because some reptiles e.g. crotalid snakes (Hartline et al. 1978) have infra-red perception and use it to locate prey in the dark, I tested whether tuatara react in any way to infra-red radiation and observed how they behave in an environment that is totally dark to the human eye. Conditions such as these can occur in the wild, e.g. deep underground burrows, caves etc.

The observations were carried out in a photographic dark-room, equipped with Kodak Wratten 6B safety lights, using a “Find-R-Scope” infra-red emitter/viewer (Lindstrom and Meyer-Rochow 1987).

Two test tuatara were not fed for three days, after which the cage containing the animals was placed in the photographic dark-room at 11.00 h. From then on all safety lights were switched off and it was totally dark. Two hours later the first test of the tuatara's reaction to the presentation of a mealworm under infra-red radiation was carried out. There was not the slightest reaction of either animal to either the beam of the infra-red light source striking them or the rapidly wriggling mealworm only 2cm in front of them. As the amount of visual pigment could possibly have been rather low at that time of day (prolonged dark adaptation results in greatly improved absolute sensitivity (Kleinschmidt & Dowling, 1975), another presentation was made at approx. 19.00 h. Once again there was no reaction.

These observations were repeated on the second and the third day of darkness - always with the same result - no reaction to the beam of the infra-red light and no reaction to the mealworm. The sound made by the mealworm when crawling in amongst dry leaves (audible to a human and no different from an illuminated worm), or its smell, were ineffective and there was no reaction to the approach of a human hand. When photographic safety lights were used instead of the infra-red light, the tuatara still did not react to the mealworm. When the cage was removed from the dark-room and exposed to ordinary daylight again, the two tuatara immediately seized and swallowed the mealworms.

We conclude from these observations that young tuatara, in all likelihood, do not use acoustic or olfactory cues, but react purely visually to potential prey. A tuatara underground or in a dark cave, therefore, is not likely to take up food even in the presence of some. The absence of any reaction whatsoever to infra-red light agrees with observations on a single, male adult tuatara made in 1973 by Wojtusiak, but their seemingly low sensitivity to light of longer wave-lengths comes as a bit of a surprise, for tuatara are regarded as nocturnal and there is, page 37 at night, relatively more light of longer (red and far-red) wavelengths around than in the daylight spectrum (McFarland and Munz, 1976). Furthermore, other reptiles like the turtle Geoclemys reevesii (Ohtsuka, 1985) show very good red sensitivity.

To some extent knowledge of the photoreceptor cell types in the retina of the tuatara could help to resolve the problem as to whether the eye of the tuatara is (a) primarily a rod-dominated receptor adapted to function as a black and white detector in dim light, or (b) a cone-dominated colour receptor originally for use in brighter light. On the basis of what is known about the tuatara retina, however, no consensus even on the cell types can be reached. Walls (1942) for example, calls a particular group of cells in the tuatara retina “rods”; Vilter (1951) calls the same cells “cones”, and Underwood (1970) after “careful scrutiny of Walls’ excellently fixed material” distinguishes major and minor single cells, double cells, and dwarf cones. The only way to solve this microanatomical riddle is by the use of an electron mcroscope, for then cones and rods can be identified without ambiguity.

Acknowledgements

The support of the New Zealand UGC for making funds available towards the purchase of equipment is gratefully acknowledged. Thanks are also given to Dr M.B. Thompson of Victoria University of Wellington and the Department of Conservation for having made available the specimens used in this study.

References

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